Order (ring theory)

"Maximal order" redirects here. For the maximal order of an arithmetic function, see Extremal orders of an arithmetic function.

In mathematics, an order in the sense of ring theory is a subring ${\mathcal {O}}$ of a ring $A$, such that

1. $A$ is a finite-dimensional algebra over the field $\mathbb {Q}$ of rational numbers
2. ${\mathcal {O}}$ spans $A$ over $\mathbb {Q}$, and
3. ${\mathcal {O}}$ is a $\mathbb {Z}$-lattice in $A$.

The last two conditions can be stated in less formal terms: Additively, ${\mathcal {O}}$ is a free abelian group generated by a basis for $A$ over $\mathbb {Q}$.

More generally for $R$ an integral domain with fraction field $K$, an $R$-order in a finite-dimensional $K$-algebra $A$ is a subring ${\mathcal {O}}$ of $A$ which is a full $R$-lattice; i.e. is a finite $R$-module with the property that ${\mathcal {O}}\otimes _{R}K=A$.

When $A$ is not a commutative ring, the idea of order is still important, but the phenomena are different. For example, the Hurwitz quaternions form a maximal order in the quaternions with rational co-ordinates; they are not the quaternions with integer coordinates in the most obvious sense. Maximal orders exist in general, but need not be unique: there is in general no largest order, but a number of maximal orders. An important class of examples is that of integral group rings.

Examples

Some examples of orders are:

• If $A$ is the matrix ring $M_{n}(K)$ over $K$, then the matrix ring $M_{n}(R)$ over $R$ is an $R$-order in $A$
• If $R$ is an integral domain and $L$ a finite separable extension of $K$, then the integral closure $S$ of $R$ in $L$ is an $R$-order in $L$.
• If $a$ in $A$ is an integral element over $R$, then the polynomial ring $R[a]$ is an $R$-order in the algebra $K[a]$
• If $A$ is the group ring $K[G]$ of a finite group $G$, then $R[G]$ is an $R$-order on $K[G]$

A fundamental property of $R$-orders is that every element of an $R$-order is integral over $R$.

If the integral closure $S$ of $R$ in $A$ is an $R$-order then this result shows that $S$ must be the maximal $R$-order in $A$. However this hypothesis is not always satisfied: indeed $S$ need not even be a ring, and even if $S$ is a ring (for example, when $A$ is commutative) then $S$ need not be an $R$-lattice.

Algebraic number theory

The leading example is the case where $A$ is a number field $K$ and ${\mathcal {O}}$ is its ring of integers. In algebraic number theory there are examples for any $K$ other than the rational field of proper subrings of the ring of integers that are also orders. For example, in the field extension $A=\mathbb {Q} (i)$ of Gaussian rationals over $\mathbb {Q}$, the integral closure of $\mathbb {Z}$ is the ring of Gaussian integers $\mathbb {Z} [i]$ and so this is the unique maximal $\mathbb {Z}$-order: all other orders in $A$ are contained in it. For example, we can take the subring of complex numbers of the form $a+2bi$, with $a$ and $b$ integers.

The maximal order question can be examined at a local field level. This technique is applied in algebraic number theory and modular representation theory.

See also

• Hurwitz quaternion order – An example of ring order

References

• Pohst, M.; Zassenhaus, H. (1989). Algorithmic Algebraic Number Theory. Encyclopedia of Mathematics and its Applications. Vol. 30. Cambridge University Press. ISBN 0-521-33060-2. Zbl 0685.12001.
• Reiner, I. (2003). Maximal Orders. London Mathematical Society Monographs. New Series. Vol. 28. Oxford University Press. ISBN 0-19-852673-3. Zbl 1024.16008.